Rotate World Normals to Camera

You can create cam normals from a world normals pass, if you have the camera. Create a ColorMatrix node and then Ctrl-Drag the first three columns of the camera’s World Matrix section into the three rows of the ColorMatrix (so, what’s top-to-bottom in the World Matrix of the cam should end up left-to-right in the ColorMatrix). That should do the trick.

(This assumes a left-handed coordinate system with green as the y-axis pointing up.)